1. Field of the Invention
The present invention relates to a method of fabricating copper plated aluminum wire and, particularly, to a method of fabricating copper plated aluminum wire in which a copper plated layer interfaced with an aluminum conductor is thermally diffused and has an improvement in adhesive properties.
The present invention relates to a plated aluminum wire, an insulating plated wire, and method of fabricating thereof. More specifically, the present invention relates to a plated aluminum wire and an insulating plated aluminum wire for use as wiring materials in an electronic device component, such as a coil, and to a method of fabricating said wires.
The present invention relates to a composite lightweight plated aluminum wire and, particularly, to a composite lightweight plated aluminum wire in which aluminum or aluminum alloy is employed as a main conductor and is used as a wiring material, such as a coil, in an electronic device.
2. Description of the Prior Art
As electronic devices and electronic components have been reduced in weight and size, their conductors, such as coils, have a smaller diameter. Also, to satisfy the demand for decreasing their weight, the conductors are selected from aluminum conductors or aluminum alloys (referred to as aluminum conductors hereinafter) of which specific gravity is substantially one-third or less that of copper. However, such aluminum conductors have electric potentials that are electrochemically unfavorable. When a metal surface of the conductor that was newly produced, e.g. by wire drawing, is exposed to air, an oxide layer is developed instantly on the surface so as to make the soldering difficult. The aluminum conductor itself has low mechanical strength and requires a particular joining technique for providing a desired degree of reliability on each joint.
Considering that the soldering of the aluminum conductors for joining is troublesome, copper-clad aluminum wires are now marketed as easily solderable, lightweight wires which are fabricated by providing a copper pipe cladding over the surface of an aluminum conductor by welding a tape of copper and drawing them to a length.
However, the copper-clad aluminum wire, when being drawn to have a desired small diameter, may often produce disconnections due to a poor adhesion at the interface between the aluminum conductor and the copper cladding.
It is also known that a plating process is favorably used for providing a higher adhesion between the aluminum conductor and the copper coat. In that case, the process of developing a closely plate coating over the surface of the aluminum conductor may be a conventional zincating technique by which a coat of zinc or zinc alloy (referred to as a zinc coat hereinafter) is tightly produced on the surface of the conductor. When the zinc coat is plated directly with a copper, a resultant copper plated aluminum wire has an improved copper plated layer.
However, during the cold plastic working process with dies, the wire is drawn several times. The adhesion at the interface between the aluminum conductor and the copper layer produced by plating is not so high. In case that its diameter is less than 0.15 mm, the copper plated aluminum wire may frequently produce disconnections.
As electronic devices have been reduced in size and weight, the demand for decreasing the weight of wiring materials, such as coils, installed in the electronic devices is increased. For satisfying the demand, simple drawn wires fabricated by copper conductors are replaced with specific copper-clad aluminum wires (referred to as copper-clad aluminum wires hereinafter) in which a coat of copper is provided on the surface of a mother material or an aluminum core (referred to as an aluminum core hereinafter) of a low density.
However, such copper-clad aluminum wires produced by the known manner are fabricated by welding a copper tape which is thus required to have a considerable thickness. This accordingly limits the increase of a ratio of cross-sectional area of the aluminum core in relation to an entire cross-sectional area of the copper clad aluminum wire for minimizing the overall weight. The maximum ratio for fabrication may be limited to about 85%. Therefore, further decrease of the weight will be difficult.
In the soldering step, the copper-clad aluminum wire of a minimal diameter (for example, 0.10 mm or less) undergoes a high degree of heat (e.g. 380.degree. C.) and may be disconnected by thermal diffusion.
When aluminum conductors are employed as wire materials in an electronic device and intended to join to corresponding terminals of the device, they have to be treated by dissolving and removing oxide layers from their surface with a heated alkali solution, neutralized using an acid, rinsed with hot water, and subjected to ultrasonic cleaning before aluminum soldering such that their joining process to the terminals will not be easy. Also, the aluminum conductors have low mechanical strength and thus, require a specific technique for ensuring the reliability of joining strength at each joint.
To overcome the difficulty of joining the aluminum conductors to the terminals, copper-clad aluminum wires are fabricated by welding a copper tape over the surface of an aluminum conductor to form a copper pipe cladding, which is slightly greater in specific gravity than the aluminum conductor, and by drawing them and marketing them as solderable, lightweight wires.
The disadvantage is that when the copper-clad aluminum wires having the copper pipe cladding provided over the surface of the aluminum conductor are decreased to a desired small diameter by drawing, they may often be disconnected due to a poor joining strength at the interface between the aluminum conductor and the copper cladding.
It is known that a plating method is preferably used for improving the adhesion at the interface between an aluminum conductor and a copper cladding. In that case, a known zincating technique may be employed for forming a plating layer directly over the surface of the aluminum conductor. A resultant layer of zinc or zinc alloy is tightly bonded to the surface. However, during the electric plating conducted directly on the zinc layer produced by the zincating technique and when the plated layer is electrically connected and immersed in a solution, the zinc layer may be susceptible to significant dissolution on the acidic side of the solution. This causes portions of the aluminum conductor to be exposed and also, of a resultant plating copper layer to have a decline in the quality by chemical displacement. This decline thus results in breakage in the drawing process.
For compensation, electroless nickel plating is conducted before the zinc plating. However, a resultant nickel plating layer has unfavorable physical properties, such as 2% of the elongation and not less than 500 Hv of Vickers hardness, and low extensibility. Accordingly, in the drawing process, the plating layer may produce lateral flaws. Also, where the electroless plating produces a plating layer which is high in extensibility and easily drawn, its speed of deposition is on the order of 3 micrometers/hour thus declining the working efficiency.